1. Field of the invention
This invention relates to a method of detecting the position of a rotor in a brushless dc motor. This invention also relates to an apparatus for detecting the position of a rotor in a brushless dc motor.
2. Description of the Prior Art
General brushless dc motors include a stator having windings and a rotor having permanent magnets. The stator windings are supplied with drive currents. To drive the rotor satisfactorily, it is necessary to sequentially change the drive currents in response to the angular relation (the positional relation) between the stator windings and the magnetic poles of the rotor.
Some of prior art apparatuses for driving a brushless dc motor include position sensors detecting the angular relation (the positional relation) between stator windings and magnetic poles of a rotor. Drive currents fed to the stator windings are sequentially changed in response to the output signals from the position sensors.
Japanese published examined patent application 61-3193 discloses a prior art advanced apparatus for driving a brushless dc motor which dispenses with position sensors. This prior art advanced apparatus uses the following fact. In a brushless dc motor, as a rotor rotates, voltages are induced in stator windings in dependence on the positional relation between the stator windings and magnetic poles of the rotor. The induced voltages are opposite to drive voltages related to drive currents fed to the stator windings. The induced voltages are generally referred to as "back" voltages. In the prior art advanced apparatus of Japanese patent application 61-3193, the induced voltages are detected as an indication of the positional relation between the stator windings and the magnetic poles of the rotor, and the detected voltages are used for the control of the change of the drive currents.
In brushless dc motors, to satisfactorily start the rotation of a rotor in a desired direction, it is necessary to preliminarily detect the positional relation between stator windings and the magnetic poles of the rotor, and then set the direction of an initial drive current in response to the detected positional relation.
The prior art position-detecting technique in Japanese patent application 61-3193 can not be used for the control of a drive current during a start of a brushless dc motor.
Japanese published unexamined patent application 63-69489 discloses a prior art rotor-position detecting system which can be used for the control of a drive current during a start of a brushless dc motor. In the prior art system of Japanese patent application 63-69489, a test current is sequentially fed to stator windings, and a variation in the test current is detected as an indication of the positional relation between the magnetic poles of the rotor and the stator windings. In this prior art system, the detection of the current-variation necessitates resistors, which dissipate a significant rate of electric power during a post-start normal operation of the motor.